High bit rate data transmission on a subscriber line is of very major importance in modern telecommunications, where there is a need to transmit both speech and data signals via the respective subscriber lines in the telephone network.
This is achieved by means of so-called xDSL technology, where DSL is short for “digital subscriber line”. With this technique, the telecommunications line, which is typically composed of copper, is subdivided into at least two different channels. As before, one of these channels is available for the conventional telephone services, that is to say for speech transmission (POTS, plain old telephone service). At least one second channel is used for data transmission.
One known representative of xDSL technology is the so-called ADSL technique (“asymmetric digital subscriber line”), which refers to a technique which allows a high bit rate bit stream to be transmitted from a control center to the subscriber, and allows a low rate bit stream from the subscriber to the control center. Since this transmission technique uses an asymmetrical bit rate, an ADSL system is particularly highly suitable for services such as video on demand, or else for Internet applications.
The ADSL method is a digital transmission method for typically twisted two-wire lines in the telephone network to the end subscriber for broadband applications. A digital signal processor (DSP) is provided for each channel, and is operated using a relatively low supply voltage of, for example, +5 V. In order to allow the signals to be transmitted at an adequate signal strength via the subscriber lines, each of the signal processors is followed by line driver circuits. A line driver circuit such as this is, in the simplest case, an amplifier which transmits the signal to be transmitted on the subscriber line with the necessary gain, since losses in the signal to be transmitted always have to be taken into account as well, during signal transmission.
These line driver circuits are subject to very stringent linearity and signal bandwidth requirements, particularly in the channel for data signal transmission, but also in the speech signal channel.
A further problem results from the fact that the amplification of the signal very frequently results in the signal to be transmitted being distorted.
There are already a large number of different circuit variants for providing a line driver circuit, some of which will be described briefly in the following text:
A Class AB amplifier is described in the article by Michael S. Kappes, “A 3-V CMOS Low-Distortion Class AB Line Driver Suitable for HDSL Applications”, IEEE JSSC, Vol. 35, No. 3, March 2000. An amplifier such as this has a relatively low efficiency of about 15% when used for ADSL, which leads to an increased power consumption in the range from 800 mW to 1 W. In order to now achieve the required gain linearity, this amplifier has to use a current which is as small as possible, although this would mean that the capability to reduce the supply voltage between the line driver and the downstream transformer is restricted. This is associated with an increase in the electrostatic charge (ESD), resulting in problems from overheating of the transformer. There may also be problems involved with technology transfer.
Furthermore, Class G amplifiers are also known, and their design and method of operation are described, for example, in the article by J. Pierdomenico, et al. “A 744 mW Supply Full-Rate ADSL CO Driver”, ISSCC 20/2, Session 19, pages 320 et seq, 2002. The efficiency of a Class G amplifier such as this is better than that of a Class AB amplifier, and this also leads to a reduced power consumption. This is because, in the case of signals with a high crest factor (Peak-to-Average Ratio=PAR), the mean output voltage is very much lower than the maximum of the output voltage. However, this is offset by very severe distortion of the transmitted signal, owing to the very frequent supply voltage switching processes that occur with this amplifier class. Furthermore, this amplifier class is very much less efficient, particularly when the transmitted signals have a low crest factor. Since, however, future systems will operate with reduced crest factors, this problem will become increasingly important in the near future.
Furthermore, Class D amplifiers also exist, which operate as PWM modulator circuits (Pulse Width Modulator). One representative of this amplifier class is, for example, described in the article by Jae H. Jeong et. al. “A Class D Switching Power Amplifier with High Efficiency and Wide Bandwidth by Dual Feedback Loops”, IEEE International Conference on Consumer Electronics (ICE '95), pages 428 et seq, 1995. Class D amplifiers admittedly have a very high efficiency in the range from 80 to 90%, but this is at the expense of very wide scatter.
In order to reduce the scatter, and hence to improve the linearity, feedback circuits are known, but these generally lead to the stability of the overall circuit being relatively poor. One such circuit with feedback is described, for example, in the Korean Patent Application No. 96/37905. A further problem associated with Class D amplifiers results from their very high switching speed. In particular, the switching speed is very much higher than the changes in the mathematical signs in the signal to be transmitted, so that amplifiers such as these are relatively unsuitable for high-speed transmission such as ADSL, since the dynamic losses rise in proportion to the switching frequency.
The already known amplifier circuits for line drivers are therefore subject to the problem that none of these amplifier circuits achieves high linearity and thus high efficiency in the signal to be transmitted with very low scatter at the same time.